1. Field of the Invention
The present invention relates, generally, to modified cellulosic fibers, to a process for preparing said fibers, and to improved cellulosic webs containing said fibers. More particularly this invention relates to cellulosic fibers characterized by a lack of swellability and incapable of natural fiber-to-fiber bonding produced by treating an aqueous slurry of the fibers with a polymeric compound, heating the treated fibers to cause the polymeric compound to react with the fibers, and refiberizing to separate individual, treated fibers. Paper products having improved properties, such as bulk and softness, absorbency are prepared from a furnish comprising these treated fibers in combination with normal papermaking fibers.
2. Description of the Prior Art
In a conventional paper-making operation cellulosic fibers are dispersed in water, drained on a wire screen, pressed into close physical contact and dried. The result is a paper sheet in which the individual fibers are held together by hydrogen bonds which give strength to the dry sheet. When the dry sheet is wet, these hydrogen bonds are broken and the paper loses most of its strength. To prevent this strength loss, various chemical treatments have been employed. Among the most successful treatments is the use of synthetic resins which, when added to the cellulosic fibers, either before or after a sheet is formed therefrom, and cured or polymerized, can significantly increase the wet strength of the sheet. Most commonly used are the urea-formaldehyde and melamine-formaldehyde type resins. These resins, because they are cationic, are easily deposited on, and retained by, the anionic paper-making fibers.
Cellulosic fibers when dispersed in water in the normal paper-making operation, absorb water and thereby swell. When formed into a sheet and pressed the fibers revert to their natural, unswollen state. In this dried condition, the fibers bond to each other through hydrogen bonding producing a stiff, compact web. It is very often desirable to produce webs which are bulkier and more absorbent than those produced via the conventional paper-making process. Such webs are used in the manufacture of sanitary products such as napkins, tissues, diapers and sanitary pads.
A low cost method of producing absorbent bulky webs encompasses the mixing of chemically modified fibers with normal, untreated fibers in the paper-making process. One way of producing these chemically modified fibers involves the crosslinking of the cellulose molecules within the fibers.
Preparation methods include for example the impregnation of cellulosic fibers with monomeric crosslinking agents, followed by heating to cause a cross-linking reaction to take place. Known techniques are identified in Shaw et al. U.S. Pat. No. 3,819,470, column 2, lines 18-28. Other methods include the treatment of cellulosic fibers with a substantive polymeric compound capable of reaction with the cellulose and/or itself. Wodka in U.S. Pat. No. 3,756,913 at column 3, lines 32-38 suggests that any of the water-soluble, thermosetting, cationic resins well-known in the art for increasing the wet strength of cellulosic sheet materials and including, for example, urea-formaldehyde resins, glyoxal-acrylamide resins, and polyamide-epichlorohydrin resins may be used for treating cellulosic fibers. Said disclosure of U.S. Pat. No. 3,756,913 might lead one of ordinary skill in the art to assume that all polymeric materials capable of increasing the wet strength of cellulosic web materials would be equally effective in producing chemically modified fibers. The present inventors, in their search for a formaldehyde-free resin capable of modifying cellulosic fibers have found that not all formaldehyde-free wet strength resins are as effective as may be desired for a commercially acceptable product. Specifically, North, in U.S. Pat. No. 4,284,758 describes a formaldehyde-free resinous product as being effective in increasing the wet strength of paper. (Column 3, lines 42-44). When the present inventors applied this resin to cellulosic fibers for the purpose of producing bulky and absorbent sheets, only a very limited modification was obtained.
Unexpectedly, the present inventors have found that a copolymer which is not thermosetting, and therefore incapable of crosslinking with itself, can be used to modify cellulosic fibers so as to render them non-bonding. Such a copolymer is completely free of formaldehyde and epichlorohydrin and cures by reaction with cellulose, an entirely different mechanism from that of the resin crosslinking with itself as in the case of the conventional, commercially available wet strength resins.